Steel sheets with surfaces plated with zinc- or aluminum-containing coatings and further treated with chromate are conventionally widely used as steel sheets for automobiles, household electric appliances, and building materials. In chromate treatment, a chromate layer is formed using a treatment solution containing hexavalent chromium (Cr(VI)) as a main component, for improving corrosion resistance (resistance to white rust and resistance to red rust). However, the chromate treatment uses hexavalent chromium (Cr(VI)) which is a pollution control substance, and thus a film subjected to non-polluting treatment not using hexavalent chromium (Cr(VI)) has recently been proposed in view of the influence on environments and human bodies. In particular, several methods using organic compounds or organic resins have been proposed. Examples of such methods are given below.                (1) Japanese Unexamined Patent Application Publication No. 63-90581 discloses a method using a thermosetting paint containing an epoxy resin, an amino resin, and tannic acid.        (2) Japanese Unexamined Patent Application Publication No. 8-325760 discloses a method utilizing the chelating force of tannic acid by using a mixed composition containing an aqueous resin and polyhydric phenolcarboxylic acid.        (3) Japanese Examined Patent Application Publication No. 53-27694 discloses a surface treatment method in which a hydrazine derivative aqueous solution is coated on a surface of a tinned or galvanized steel sheet.        (4) Japanese Unexamined Patent Application Publication Nos. 2002-53980 and 2002-53979 disclose a technique for forming a phosphoric acid and/or phosphoric acid compound film containing an oxide as a lower layer, and then forming an organic composite coating comprising a resin film as an upper layer.        
However, these conventional techniques have the following problems:                Any one of methods (1) to (3) causes insufficient corrosion resistance due to the fact that the resulting film has no self-healing effect.        The corrosion resistance of a chromate film is exhibited by the synergetic effect of a barrier effect and a self-healing effect. The barrier effect is a barrier effect against corrosion factors (water, oxygen, chlorine, and the like) of a slightly soluble compound (hydrous oxide) mainly composed of trivalent Cr. The self-healing effect is the effect of forming a protective film by hexavalent Cr (Cr(VI)) at a corrosion origin.        In chromium-free techniques (1) to (3), the barrier effect can be imparted to some extent by an organic resin or the like without using chromium. However, the self-healing effect cannot be exhibited by hexavalent Cr (Cr(VI)), and thus a high degree of corrosion resistance cannot be realized.        On the other hand, in technique (4), the corrosion resistance is improved to some extent by adding a specified substance exhibiting the self-healing effect to the upper layer. However, the corrosion resistance is not necessarily sufficiently improved. This is because the film formed at the interface between the zinc coating and the upper layer, i.e., the lower layer, does not contain hexavalent Cr (Cr(VI)), thereby failing to obtain a direct rust-proofing effect due to the self-healing effect.        
Accordingly, it could be advantageous to provide a surface-treated steel sheet exhibiting excellent corrosion resistance, excellent conductivity, and excellent coating appearance even when the film does not contain a pollution control substance such as hexavalent chromium (Cr(VI)).